6.5.2 Cross-polar interference cancellers (XPICs)
Co-channel frequency re-use can be seriously affected by co-frequency cross-polarized interference arising from low XPD values that may occur during periods of multipath fading. The development of antennas with improved XPD characteristics about the boresight may in some circumstances provide co-channel frequency re-use operation. Where additional reduction of cross-polar interference is required, an adaptive canceller (XPIC) may be used in such situations to cancel a cross-polar interference signal from an orthogonal polarized signal.
The XPIC takes samples of the interfering signals, which are available in the cross-polarized channel, and feeds them through complex coupling networks to the desired channel to cancel the interference. The control of the added signal and the compensation of the interference can take place at RF, IF or baseband levels. Transversal filter structures are also used for XPICs to compensate effectively a cross-polar interference signal.
With higher multi-state modulation, such as 256-QAM, a highly precise interference canceller is required to provide adequate equalization performance. Propagation experiments have shown that receiving systems equipped with an XPIC lead to considerably reduced outage times at a BER of 103.
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